An essential part of electrostatographic printing systems widely used for office copiers, computer print outs, data retrieval, etc., is the electroconductive coating composition used to improve the electrical conductivity of the substrate reprographic paper. Paper has a surface electrical resistivity (SER) greater than about 10.sup.14 ohms/square at 10% relative humidity (RH). But most electrophotographic copiers require a maximum SER of about 10.sup.10 -10.sup.11 ohms/square. Electrographic printing also requires a similar lower SER.
Particularly effective electroconductive additives for paper are water-soluble quaternary ammonium polymers such as the poly(vinylbenzyltrimethylammonium chloride) resin used in Silvernail and Zembal, U.S. Pat. No. 3,011,918. Other quaternary ammonium electroconductive resins include the poly(diallyldimethylammonium chloride) resin of Boothe and Hoover, U.S. pat. No. 3,544,318, the quaternized polyethylenimine of Cavagna and Walker, U.S. Pat. No. 3,479,215, and the quaternized polyepichlorohydrin of Rogers and Woehst U.S. Pat. No. 3,320,317.
These electroconductive resins are seldom applied neat to the base substrate. Rather they are typically applied as a formulated, aqueous coating composition containing the electroconductive resin, a film-forming binder such as starch, polyvinyl alcohol or a latex, and a suitable pigment such as clay, calcium carbonate, etc. The type and amount of binder has a pronounced effect not only on SER, but also on resistance to organic solvents used in subsequent processing. The solvent resistance or "holdout" to aromatic solvents used in coating the photosensitive ZnO layer and to aliphatic solvents used in liquid toning is important. The pigment has less effect on conductivity, but influences the stiffness, tack, gloss and brightness of the reprographic sheet.
Although starch is commonly used as a binder with quaternary ammonium resins, it provides only moderate solvent holdout and at high concentrations has a detrimental effect on resin conductivity. With the poly(vinylbenzyltrimethylammonium chloride) resin, polyvinyl acetate provides high conductivity but poor solvent resistance while polyvinyl alcohol gives high solvent resistance and poor conductivity. Methyl cellulose as a binder provides good conductivity and solvent resistance but is costly.
Paper has long been sized with such materials as rosin and starch to improve its resistance to grease, oil and water. Sizing of paper with certain acrylamide polymers is described by Lehman, Silvernail and Stilbert U.S. Pat. No. 2,945,775. Azorlosa U.S. Pat. No. 2,616,818 and U.S. Pat. No. 2,661,309 disclose other paper coating compositions containing acrylamide copolymers as a pigment binder to provide improved brightness, smoothness, gloss and feel.